Metallurgical silicon is generally obtained by reduction of silica using carbon in a submerged arc electric furnace. The silicon produced in the liquid state is then cast in ingots whose weight may range from 10 to 1,000 kg. After solidification and cooling, the ingots may be crushed, then ground into a powder.
The grinding mills work most often in the open air. However, for reasons of safety, they may be placed in a reduced-oxygen atmosphere, e.g., one containing 5-10% O.sub.2. In fact, the mixture of air and powdered silicon in the mills may entail risks of explosion.
To avoid the grinding operation, the liquid silicon may be atomized, thus directly producing a powder composed of coarsely spherical particles. This technique also makes it possible to reduce the size of the finest granulometric fractions and to ensure very rapid solidification, so as to promote formation of special crystalline structures.
These various methods make it possible to obtain various types of silicon differentiated by their crystalline structure, which depends on the solidification and cooling rate used. Different types of silicon may also be produced by adjusting the proportions of impurities and/or alloyed elements added in small quantities.
While the preparation method and composition allow silicon grades suitable for the intended use to be obtained, it emerges that surface oxidation on the particles forming the powder of said Si is still observed. This oxidation exists as a silica film covering the entire surface of said particles, and its thickness normally ranges between 3 and 5 nm (30 and 50 .ANG.) but is virtually never less than 3 nm (30 .ANG.). It may happen that, on rare occasions, thicknesses of silica of less than 3 nm (30 .ANG.) are measured, but the processes for obtaining the Si powders described above do not make it possible to obtain such powders exhibiting low oxidation regularly and in a reproducible fashion, nor to ensure their surface quality.
In its applications, metallurgical silicon is often used as a raw material in heterogenous reactions, e.g., in the presence of a gas like methyl chloride for production of dichlorosilane in order to produce silicones, or in the presence of a liquid such as molten aluminum in order to manufacture an Al--Si alloy.